1. Field of the Invention
The invention relates to an optical distance measuring device and process in a web production machine, e.g., a paper or board machine, for measuring a parameter of the web, e.g., web thickness, surface roughness, etc. More particularly, the invention relates to an air bearing supported optical distance measuring device measuring a distance to the web.
2. Discussion of Background Information
Known optical distance measurement in the micrometer range is generally based on either laser triangulation or confocal principle. Laser triangulation is accomplished by a device projecting a small spot of light onto a surface. A lens having an optical axis oriented at an angle to the axis of the projected light forms an image of the spot onto a position sensitive detector. As the surface moves closer or farther from the device, the image of the spot moves, such that the output of the position sensitive detector is used to determine the distance from the device to the surface.
In accordance with the confocal principle, a confocal sensor light from a source, typically a laser, passes through a beam splitting mirror and is focused onto an object through a lens. Light scattered back from the object returns through the same lens, is separated from the source beam by the beam splitting mirror and is focused onto a pinhole. A detector behind the pinhole collects light that passes through it.
When the object is at exactly the focal point of the lens system, the image of the laser spot is tightly focused on the pinhole and the detector collects the maximum possible light. When the object moves away from the focal point, the image of the laser spot at the pinhole spreads out, such that less light passes through the pinhole to the detector.
Distance is measured by moving the pinhole and finding the distance at which the maximum amount of light penetrates the pinhole. Alternative methods for measuring distance are also known, e.g., as described in U.S. patent application Ser. No. 11/108,337, the disclosure of which is expressly incorporated by reference herein in their entirety.
However, a difficulty arises with optical devices using the coherent light from a laser diode. Coherence is the property of light that enables it to exhibit interference. To be coherent, a light source must be monochromatic and emit light waves in phase. In theory, perfectly coherent light will remain coherent until disrupted. Light emitted from a nearly coherent source will lose its coherence after some distance. The distance light travels before it loses its coherence is the coherence length. Lasers emit light with a high degree of coherence. Even small amount of coherent stray light reflected and/or scattered from the source can interfere with the measurement of the relatively small amount of light backscattered from the object being measured. Also, very small amounts of light reflected back into the laser diode can cause significant interference with the functioning of the laser diode. Of particular difficulty are reflections from optical surfaces such as windows, beam splitters and lenses because the reflected light often follows the same path as the light to be measured. While anti-reflection coatings reduce reflected light to a fraction of a percent, with an intense source even this fraction is sufficient to cause problems. The above-noted conventional confocal microscope mitigates the effect of stray light to some extent by using a tiny pinhole at the focal point of the collecting lens. However, reflected light traveling the same path as the light being measured is allowed through the pinhole into the detector and can interfere with the measurement.
It is likewise known to utilize an optical device to observe surface topography at high speed (several MHz) in order to measure roughness. In the paper industry, surface roughness is a parameter relating directly to print quality. High quality printing grades go through coating and calendering steps to improve roughness and therefore print quality.
Further, the thickness (caliper) of a sheet can be measured with two (2) optical devices located on opposite sides of the sheet and separated by a known distance. If the distance between the optical devices is not constant, it can be measured with an inductive device that is not sensitive to the presence of the sheet. Thickness is calculated by subtracting the measured distances from the two optical devices to respective surfaces of the sheet from the distance between the optical devices.
Moreover, it is known to use an air bearing supported sensor to follow the sheet, but such sensors must be very small and light. However, this small size results in a light source located very near the sheet, such that the resulting optical light path to be measured is very short. Also, a significant fraction of the light reflected from the sheet and the surfaces of the window can be traveling back into the light source. If the source is a laser diode, this optical feedback is still coherent with the outgoing beam and can cause significant changes in the output amplitude.
With a fixed distance from the source to the window and a variable distance to the sheet, the two reflections from the window will interfere with the reflection from the sheet in such a way that the interference will be either destructive or constructive depending on the distance from the window to the sheet. Thus, the intensity of the light reflected back into the light source will not be constant, making the feedback effect very serious. Periodic variations are caused by the above-noted optical feedback, and the period of the error curve is approximately half of the wavelength used. If a laser is used as a source with a window close to the sheet, then, to reduce periodic variations, the reflections back to the laser must be reduced to a very small percentage of the output intensity. The reflection from the inner side of the window will be attenuated naturally because it is relatively far from the focal area near the sheet, but the outer side of the window is very close to the sheet and the reflection from that surface will almost inevitably end up going back to the light source, particularly because the sensor has been miniaturized so that it can be supported by an air bearing.
Methods have been devised in an effort to limit this effect. Such methods include using a very small source aperture or adding a strongly attenuating neutral density filter to the source beam. While both of these methods reduce the feedback effect, they also reduce the available signal intensity significantly. Other possible methods use optical surfaces that are either not flat or not in a right angle relative to the optical axis of the sensor. These methods complicate the design, and the outer side window surface, which is most likely to cause the problem, should be flat and parallel with the sheet, i.e., it will be at least nearly perpendicular to the optical axis.
An anti-reflection coating on the outer side of the sensor window will not last because it will occasionally be scraped off by the moving sheet of a paper machine. A fast flutter of the paper sheet combined with the inertia of the sensor can overcome the force of the air bearing allowing the sheet to scrape occasionally. Cleaning the surface would also eventually destroy the coating.